As a method for measuring particulate matters (PM) as being one of exhausted materials from an engine well known is a filter mass method that collects particulate matters by the use of a filter and measures a mass of the particulate matters. However, since the exhausted amount of the particulate matters has been subtle, it becomes difficult for the filter mass method to measure the amount in view of the accuracy. Under this condition, as its alternative method developed is a method for measuring a number of the particulate matters in the exhaust gas. A concrete system structure is known that a diluter to dilute the exhaust gas of the engine with air or the like is arranged upstream of a particle number measuring unit, and a part of the diluted exhaust gas is introduced into the particle number measuring unit and then the number of the particles contained in the diluted exhaust gas is counted (refer to the patent document 1).
Conventionally, the dilution unit of this system comprises a diluter arranged at a connecting point of the main flow channel where the exhaust gas flows and the dilution gas flow channel where the dilution gas flows or near downstream of the connecting point, a flow rate measuring mechanism to measure a mss flow rate of the exhaust gas introduced into the diluter, a dilution gas flow rate control part to control the mass flow rate of the dilution gas introduced into the diluter and an exhaust gas flow rate control part to change the mass flow rate of the exhaust gas. Then a desired dilution factor is realized by measuring the flow rate of the exhaust gas flowing into the dilution unit by means of the flow rate measuring mechanism and by controlling the flow rate of the exhaust gas by means of the exhaust gas flow rate control part.
The flow rate control mechanism comprises an orifice part as being a fluid resistance, a pressure sensor that measures a differential pressure of the orifice part, and a pressure sensor that measures an absolute pressure in the upstream side, and is so arranged that an information processing unit, separately arranged, calculates a mass flow rate of the exhaust gas introduced into the diluter based on the pressure information upstream and downstream of the orifice part.
However, if the orifice part, the pressure sensor for measuring the differential pressure, the pressure sensor for measuring the absolute pressure and a temperature adjustor are arranged in the upstream side of the diluter, there are not only a problem that the system for determining the number of particles becomes bulky but also a problem that a number of components increases so as to increase its cost.
In addition, the system for determining the number of particles has an arrangement that a bypass flow channel is arranged between the dilution unit and the particle number measuring unit, air whose flow is controlled by a mass flow controller is supplied to the bypass flow channel and the flow rate of the exhaust gas introduced into the particle number measuring unit from the dilution unit is adjusted.
However, since this arrangement uses the mass flow controller, there are problems such that not only the system for determining a number of particles becomes bulky but also its cost increases.